Ferrous-base, hardenable, corrosion-resistant, high-strength, high-ductility alloy



United States Patent 3 285,738 FERROUS-BASE, HARDENABLE, CORROSION-RESISTANT, HIGH-STRENGTH, HIGH-DUC- TEITY ALLOY Thomas E. Johnson,Milwaukee, Wis., assignor to Stainless Foundry & Engineering, Inc.,Milwaukee, Wis., a corporation of Wisconsin No Drawing. Filed May 22,1964, Ser. No. 369,625

3 Claims. (Ci. 75-128) This invention relates to a copper-free,ferrous-base, hardenable, corrosion-resistant, high-strength,high-ductility alloy. More particularly, the invention relates to alloysof the stainless steel type composed primarily of iron, which aresubstantially free of copper, but which include a substantial amount ofchromium and smaller amounts of other elements, including specificamounts of cobalt, which are in such relative proportions as to imparthigh corrosion resistance, high strength and high ductility. The termscopper-free or substantially free of copper or similar terms used hereinare intended to mean less than 0.50 percent by weight of copper.

The chemical processing, food processing, and pharmaceutical industries,in handling chemicals and solutions of corrosive materials, such asacids and alkalies, is beset by many difiicult requirements as toequipment and parts in contact with the corrosive environment. Suchequipment and parts must be constructed of a metallic material having ahigh resistance to corrosion, and further, should corrosion occur, anymetallic salts transmitted to the product by the decomposition of themetal must be such a nature that they will not be deleterious to theproduct. Trace contamination by metallic salts of the element copper isknown to be undesirable in certain processing. For example, in themanufacturing of food products from vegetable oils, copper orcopper-containing alloys are not generallyused because tracecontamination of the food product by copper salt products of corrosioncan result in noxious discoloration which renders the product worthless.

Equipment and component parts, such as pumps, impellers, shafts, valves,pipes, bearings, pipe fittings, vessels, tanks, and the like, must bemanufactured of a metal possessing adequate corrosion resistance butother desirable properties should be provided by this corrosionresistant material. Such properties of strength, hardness in conjunctionwith ductility, machinability, and abrasion resistance are highlydesirable. Economy in the design of equipment and a reduction in theweight of equipment can be realized through utilization of high strengthalloys. Abrasion resistance is provided by alloys possessing acombination of high hardness, high ductility, and high yield strength.

Austenitic chromium nickel stainless steels such as the 18% chromium, 8%nickel grade, Type 304 or the 18% chromium, 12% nickel, 2% molybdenumgrade, Type 316, are extensively used in the manufacture of equipmentfor the chemical, food, and pharmaceutical industries. These materialspossess a high degree of ductility but are relatively soft and aredeficient in hardness and strength for many applications.

The element copper is frequently used in the formulation of specialtygrades of stainless steels possessing both high hardness and highstrength. The strengthening of these specialty grades of stainless steelis accomplished 3,285,738 Patented Nov. 15, 1966 by thermal treatmentwhich results in the strengthening of the alloy matrix through aphenomenon known to the art as precipitation-hardening. The precipitatedphase in certain of these alloys has been identified as a copper richphase, possibly pure copper. When such copper containing alloys areexposed to corrosive solutions the possibility of product contaminationby copper compounds exists, as minute particles of a copper-richmaterial are exposed to the corrosive environment. 7

It is the object of this invention to provide hardenable iron-basealloys in which chromium, nickel molybdenum, and cobalt are used inrelative amounts to obtain optimum combination of properties including ahigh degree of corrosion resistance, high strength and hardness, and ahigh degree of ductility.

Another object of this invention is to provide hardenable iron-basecorrosion resistant alloys containing definite related proportions ofchromium, nickel, and molybdenum, with cobalt therein in selected andrelated amounts to provide a high degree of ductility together with highmechanical strength.

Still another object of this invention is to provide a hardenable ironbase alloy containing little or no copper, to provide an alloy which maybe utilized in corrosionresisting environments where trace contaminationof copper is detrimental.

A further object of this invention is to provide an iron base corrosionresistant alloy which can be softened by thermal treatment to render itreadily machinable and one which can subsequently be hardened by thermaltreatment to provide a combination of desirable properties includinggood corrosion resistance, high strength, high ductility, and highhardness Another object of this invention is to provide a cor rosionresistant iron base alloy of high strength and high hardness which haslow brittleness and high ductility as compared to prior art alloys, andcan be fabricated by casting, forging, machining and welding intoimplements used to resist corrosion.

Various other objects will be apparent from the following descriptionand claims.

In accordance with the present invention I have discovered thatadditions of cobalt in certain high chromium steels, without theaddition of copper, results in steels which have very high strength andare ductile. The resultant alloy is capable of being strengthened by aprecipitation-hardening heat treatment. One advantage of my alloy overexisting commercial alloys is that it is essentially a copper-freematerial. Prior art commercially available alloys, such as 17-4PH,CD4MCu and V2B employ copper to gain precipitation-hardening properties.In some industries, such as the food or pharmaceutical industries,copper-bearing alloys have limited application. Furthermore, coppermakes such alloys prone to cracking in heat treatment, especially whenheavy sections of such alloys are hardened. The use of specific amountsof cobalt, according to this invention, in combination with relatedproportions of chromium, nickel and molybdenum in an iron-base alloyprovides an alloy which can be readily fabricated by the usual casting,forging and welding operations, which can be softened by thermaltreatment to render it readily machinable and which can be hardenedsubsequently by thermal treatment to provide a combination of desirableproperties including good cor- In accordance with this invention, I havefound. that the alloy should have the following composition:

range are more ductile than existing alloys at a given strength level.

Table II Analysis, Weight Percent Corrosion Test Data Type Heat No. SaltSpray (1) Boiling 65% Nitric Car- 8111- Manga- Chro- Nickel Molyb- Cop-Cobalt (Mg, Weight Vinegar (2) Ac bon eon nese mium denum per Increase)(Rate, (Rate.

-p-y ay) 31688 B-660 .09 1.48 .37 19 .92 9.90 2.00 0.22 Nil CD4MCuT-942- .04 .43 .61 26.00 6.04 2.15 3.40 Nil Copper Free E-069. .07 .521.13 25.30 5.50 2.30 0.08 0.95

Copper Free E-070- .07 .51 .61 24.96 5.50 2.30 0.10 5.08

Copper Free. 13-071-.- .08 .52 .65 25.44 5.58 2.30 0.06 8.08

Copper Free 15-072.... .07 .52 .64 25.90 5.50 2.30 0.06 12.00

In accordance with this invention, I have found that the alloy shouldhave the following composition:

PERCENT BY WEIGHT 1 Balance, except for residual elements and impuritiesin amounts up to .050%.

The following table designated Table I gives examples of heats of alloyswithin the series covered by this inven- Corrosion tests were made onheats of alloys coming within this present invention and an alloy 31658,a substantially copper-free and cobalt-free alloy, as well as on thecopper-containing alloy CD4MCu. Table 11 above is a tabulation of theresults of these tests.

To obtain the data in Table II, three basic tests were involved:

(1) A 96-hour salt spray test as described in the American Society forTesting Materials Standard No. B-117. In this test all materials testedexhibited a slight gain in weight indicating the build up of a passivecorrosion resistant film. The copper free alloys of this invention wereequal to the Type 316 stainless steel and copper bearing CD4MCu alloytested. There was no evident corrosion of any of the materials tested.This test series demonstrates the ability of the alloy of this inventionto withstand corrosion of salt water and salt water atmospheres.

tion which are substantially copper-free. These alloys 40 (2) A 48-hourtest in boiling vinegar indicates the alloy are compared in the tablewith a copper-free prior art of this invention to be equal to Type 316stainless steel alloy designated HC, with a substantially copper-freeand the copper bearing CD4MCu alloy. No measurable alloy that does notcontain cobalt designated B868, and corrosion was encountered in any ofthe alloys of this also with a copper-containing alloy designatedCD4MCu. invention which were tested. This test series demon- Table IB868 E069 B867 E070 D713 E493 E071 E072 Type HG OD4MCu ChemicalAnalysis, Percent:

Carbon 0. 11 0. 07 0. 06 0. 07 0.08 0. s 0. 0s 0. 07 1 0. 1 .04 Chr0mium24.74 25.30 25.34 24.96 26.29 25.46 25.44 25.90 26-30 25.27 Nickel 6.145. 50 6.00 5. 50 5. 86 5. 40 5. 53 5. 50 1 4. 0 4. 75-6. 0 Molybdenum 2.50 2. 30 2. 2. 2.25 2.15 2.30 2. 30 1.75 2. 25 Cobalt Nil 0. 95 4. 82 5.03 5.12 7. 23 3. 03 12. 00 Silicon 0. 0. 52 0. 40 0. 51 0. 24 0. 52 0.52 0. 52 1 2. 0 1 1. 0 Manganese 0. 1. 13 0. 53 0. 61 0. 41 0. 46 0. 650. 64 1 1. 0 Copper 0. 23 0.08 0. 32 0. 10 0.12 0.10 0.06 0. 06 2. -3.25 Mechanical Properties:

Solution Heat Treated (Note 1):

Yield strength, p.s.i 78, 200 76,200 75, 000 72, 000 68, 500 70, 000 66,62, 800 76, 000 50-90, 000 Tensile strength, .s.i 104,400 99,100 103,10099, 900 98, 100 99, 300 99, 100 100,700 110,000 100-115, 000 PercentElongation in 22. 0 36. 0 33. 0 35. 5 34. 0 37. 0 41. 0 41. 0 10 20-30Brinell Hardness 212 212 212 217 207 217 201 197 223 250-270 Solutionedand Aged (Note 2):

Yield strength, p.s.i 90,000 90, 000 95,200 83,000 86,500 89,500 79,50074,000 80,000 1012120, 000 Tensile strength, p.s.i 122, 000 116,000 131,500 119, 500 123,500 125,500 120,000 120,000 115, 000 -145, 000 PercentElongation in 2" 1e. 5 23. 5 27. 5 29. 5 31. 31. 0 36.0 23. 5 10-25Brinell Hardness 255 255 255 255 255 241 255 241 29(2320 Max. News:

1 Solution heat treated by holding at 1,900" F. to 2,050 F. for one hourand cooling rapidly by quenching in Water.

2 Aged by thermal treatment at 850 F. for (4) hours.

It will be apparent from a study of the table that the following factsare emphasized:

(1) That the alloy of this invention is substantially copper-free and isless than the allowable maximum of 0.50 percent.

(2) That the addition of cobalt within the range previously set forthimparts age-hardenable properties and increased ductility not obtainablein alloys free of cobalt.

(3) That alloys containing cobalt within the required strates theability of this alloy to withstand the corrosion of a typical severeenvironment encountered in the food industry.

(3) A 48-hour test in 65% boiling nitric acid per the American Societyfor Testing Materials Standard No. A262 indicates the new copper-freealloy of this invention to be substantially better in this media thanType 316 stainless and equal to CD4MCu. This is an example of anindustrial environment.

(4) Corrosion ratesrnills per year.

In an experiment to demonstrate that the solution heat treatment, duringage-hardening, of this corrosion resistant and mechanically strongalloy, would not produce quench-cracking, test castings having two 3" X2%" and 2 /2" block sections connected by a A" thick Web were made.These castings from heat No. E-493, were water quenched from 2050 F. andsubsequently age-hardened at 900 F. The samples were then sectioned andexamined for cracking by utilizing a dye-check inspection process. Nocracking was evident. Thus, the copperfree alloy of this invention isresistant to quench cracking and thermal shock. Apparently thepermissible small copper content, that is, less than 0.50%, remains insolution and is not precipitated upon heat treatment.

It will be apparent that this invention provides an alloy of thestainless steel type which is corrosion resistant, of high-strength andhigh-ductility. By the use of specific amounts of cobalt, in combinationwith substantial amounts of chromium and smaller amounts of otherelements, it is possible to provide a substantially copperfree alloywhich can be heat treated and age-hardened to provide desirablemechanical properties.

Although some of the advantages of this alloy have been discussed above,others will be apparent.

Having thus described this invention, what is claimed 1. An alloy inpercent by weight consisting essentially of:

Carbon 0.03- 0.12

Chromium 24.00-28.00 Nickel 4.00- 6.50

Cobalt 0.95-12.00 Molybdenum 1.75- 3.00

"-"'. T.-""T"T'T'TT"- T-? TC'-'7- TT Manganese Copper Iron, balance andincidental impurities.

2. An alloy in percent by weight consisting essentially Iron, balanceand incidental impurities.

3. An alloy in percent by weight consisting essentially of:

Carbon 0.06 Chromium 26.00

Nickel 5.50 Cobalt 6.50 Molybdenum 2.25 Silicon 0.50 Manganese 0.50Copper Nil Iron, balance and incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 2,306,622 12/1942 Krivobok 128 2,518,715 8/1950 Payson 75128 X DAVID L. RECK, PrimaryExaminer. P. WEINSTEIN, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,3,285,738 November 15, 1966 Thomas E. Johnson It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 3, lines 1 and 2, strike out "In accordance with this invention,I have found that the alloy should have the following composition:" andinsert instead rosion resistance, high strength, high ductility and highhardness.

Signed and sealed this 19th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD]. BRENNER Commissioner ofPatents

1. AN ALLOY IN PERCENT BY WEIGHT CONSISTING ESSENTIALLY OF: